1. Field of the Invention
The present invention relates to a scanning optical apparatus, and particularly to an scanning optical apparatus suitable for an apparatus, such as a laser beam printer using an electrophotography process or a digital copying machine, in which a light flux emitted form light source means is deflected by a deflecting element to scan a surface to be scanned with light via an imaging element having fθ characteristics so as to record image information. The present invention also relates to an image forming apparatus using such a scanning optical apparatus.
2. Related Background Art
In conventional scanning optical apparatuses such as a laser beam printer (LBP), a light flux emitted from light source means is optically modulated in accordance with an image signal. The light flux thus optically modulated is periodically deflected by a light deflecting device composed of a polygon mirror and focused by an imaging optical system having fθ characteristics onto a surface of a photosensitive substrate as a spot, so that image recording is carried out by optical scanning.
FIG. 11 schematically shows a conventional scanning optical apparatus (see Japanese Patent Application Laid-Open No. H10-148781, Japanese Patent No. 3365869 or Japanese Patent Application Laid-Open No. 2001-305459). In FIG. 11, a diverging light flux emitted form light source means 1 is converted into a substantially parallel light flux by a collimator lens 2 and made incident on a cylindrical lens 4 while regulated by a stop 3. The parallel light flux incident on the cylindrical lens 4 is emitted from it without being changed with respect to the main scanning plane, but focused with respect to the sub scanning plane, so that the light flux is imaged on a reflecting surface of a deflecting element 5 composed of a polygon mirror as a substantially linear image. The light flux reflected and deflected by the surface of the deflecting element 5 is guided to a surface to be scanned 8 via a scanning optical element (an fθ lens) 6 having fθ characteristics. The surface to be scanned 8 is scanned by rotating the deflecting element 5 in the direction indicated by arrow A.
A scanning optical apparatus for use in a color image forming apparatus is provided with a plurality of light source means and light fluxes are made incident on one or multiple deflecting elements. The plurality of incident light fluxes have angles relative to the deflection scanning plane and a plane including an optical axis of the deflecting element and orthogonal to the deflection scanning plane respectively. After passing through the one or plurality of scanning optical elements, the light flux is split by a mirror or the like and a plurality of different surfaces to be scanned are scanned by a plurality of light spots.
Conventionally, in the scanning optical apparatus and the image forming apparatus using the same, there has been a problem that deterioration of image quality such as streaks in an image of doubled images occurs on account of ghost light generated from optical elements or mechanical elements in the scanning optical apparatus.
In recent years particularly, with use of plastic materials for scanning optical elements or cylindrical lenses, lenses without anti-reflection coating have become mainstream, and therefore significant amount of ghost light is generated at lens surfaces. In addition, with downsizing of the scanning optical apparatus, a chassis for supporting optical elements and an optical path have become spatially close to each other. Consequently, ghost light generated at mechanical members such as a chassis, a stop or a non-mirror portion of a deflecting element has been increasing.
A so-called tandem type color image forming apparatus has four photosensitive members in which light scanning apparatuses are provided for the respective photosensitive members so that latent images are formed by laser light and images of respective colors of Y (yellow), M (magenta), C (cyan) and Bk (black) corresponding to an original are formed on the respective photosensitive members. The tandem type color image forming apparatus is equipped with a scanning optical system that scans light fluxes in the opposite directions by a common deflecting device, and it suffers from a problem that generated ghost light can be incident on the opposite surface to be scanned to cause streaks in images or misregistration of colors.
As measures for reducing such ghost light, there has been known:
(1) blocking the ghost light with a mechanical member (disclosed in Japanese Patent Application Laid-Open No. H10-148781);
(2) separating the ghost light utilizing eccentricity of an optical element (disclosed in Japanese Patent No. 3365869); or
(3) guiding the ghost light outside the effective image area (disclosed in Japanese Patent Application Laid-Open No. 2001-305459) However, (1) in the case that the ghost light is blocked by a mechanical member, requirement for precision in position of the mechanical member is strict and there is a risk that an image writing light flux is also blocked in some cases, (2) in the case that the ghost light is separated by eccentricity of an optical element, aberrations of the optical system are deteriorated by the eccentricity and curvature of scanning line and rotation in the spot shape are caused, and (3) the optical design in which the ghost light is guided outside the effective image will reduce freedom in design and the structure will be complicated. In addition, with any one of the above measures, there remains ghost light that cannot be eliminated completely depending on the shape of optical elements, for example, ghost light generated by internal reflection.